The present invention relates to a new method for evaluating insulating films by forcibly flowing an electric current through an insulating film to easily measure or estimate a time until the insulating film is broken down.
1. Field of the invention
The present invention relates to a method for evaluating insulating films according to a distribution of breakdown current densities of an insulating film. This method is as simple as the conventional method of finding a distribution of insulating strength, and yet makes it possible to evaluate the film quality of the insulating film more accurately than the conventional method for of finding the distribution of insulating strength. Further, this method has a close correlation to the constant-current TDDB of Time Dependent Dielectric Breakdown, and makes it possible to accurately estimate a breakdown time of the insulating film without a long period of time to measure.
2. Description of the Prior Art
Conventional typical technique for evaluating insulating films includes a voltage ramping method and a TDDB method. According to the voltage ramping method, a voltage applied to the film is gradually increased in order to measure a voltage or an electric field when a current has exceeded a predetermined value. According to the TDDB method, a constant voltage is continuously applied to an insulating film or a constant current is continuously supplied into the insulating film to measure a time until the insulating film is broken down.
The above conventional methods are disclosed in the following papers:
A. Berman, "Time-zero dielectric reliability test by a ramp method," IEEE/IRPS, pp 204-209, 1981 PA1 D. L. Crook, "Method of determining reliability screens for time dependent dielectric breakdown," IEEE/IRPS, pp 1-7, 1979
In the conventional ramping method, there are such problems as follows; Progress in the materials and equipment in recent years has contributed to improve the quality of insulating films so that pinholes and weak spots are contained in small amounts. Therefore, the voltage ramping method is no longer capable of obtaining good results, and it is becoming difficult to evaluate the film qualities minutely.
FIG. 3 illustrates distributions of insulating strength of SiO.sub.2 films having thicknesses of 100 to 220 angstroms. Though these films have different qualities which were produced by different heat treatment, hardly any difference between them can be recognized from FIG. 3.
The TDDB method, on the other hand, is effective to evaluate the quality and reliability of the insulating films, but requires a long period of time since the time must be measured until the film is broken down.
FIG. 4 illustrates the distribution of insulating strength of SiO.sub.2 films according to the TDDB method, and evident differences can be recognized from FIG. 4. Moreover, since a number of specimens must be measured to find statistical results, a greatly extended period of time is required using the TDDB method.
In recent years, a constant-voltage TDDB method has been introduced which is capable of measuring many points or spots simultaneously. However, this method is not able to detect the total amount of electric charge that flows through the film. A constant-current TDDB method, on the other hand, is capable of easily detecting the total amount of electric charge, but is quite difficult to simultaneously measure a number of points. Therefore, this method requires a greatly extended period of time.